Please understand that we are all time travelers. As this paper is being read, the reader will have traveled from one place in space time to another. This author's intent is to speak of time travel above and beyond that which is naturally achieved on Earth. Before we delve further into the possibility of time travel the author believes that it would be beneficial to actually define time. Hawking states that there are only three ways time can be seen to move. These he calls arrows of time (145).
The first is the thermodynamic arrow of time. The second law of thermodynamics states that the entropy (disorder) of the universe can only increase and at best stay the same. Imagine a picture of a wine glass sitting on a table and one of another lying shattered on the floor (Ferguson 77). Common sense tells us which event preceded the other. The second law of thermodynamics tells us that the more orderly picture must have come before the less orderly one. The energy expended to reform the shattered glass increases the entropy of the universe as much as or more than the finished product decreases it.
Secondly, there is the psychological arrow of time. This consists of our own personal view of the movement of time. Boring or undesirable activities seem to take more time than the more enjoyable ones. As a deadline nears time seems to speed up. This arrow of time allows us to remember the past but not the future. Also, human beings tend to think of time as circles. One season changes to another until we're back where we started from (Davies 257).
Lastly, we have the cosmological arrow of time. This arrow of time tells us that the universe is expanding rather than contracting. Many have ask why we should be around during the expansion faze of the universe. Hawking believes that we ask this question because this is the only time the universe can produce intelligent life (147). During the contraction phase of the universe, the entropy would be so high that life could not produce enough order to capture the energy given off when it becomes disorderly. Essentially, we use plants and animals to convert the raw environment into ordered matter. We then consume this, digest it, and give it off as heat.
Traveling through time can come under two main headings: nonphysical and physical. Physical time travel consists of transporting a person, space ship, object, etc. through time into the past or future. This can be very complicated and will be talked about in detail at a later time. (No pun intended.) However, some believe that nonphysical time travel can be achieved by almost anyone, anywhere, anytime. The simplest way is to look into your dreams.
J. W. Dunne found that if he carefully recorded his own dreams he could find a correlation between them and real events that would happen later on in his life (Randles 10). In her book, Time Travel: Fact, Fiction and Possibility, Randles tells us how she has used this technique to predict the future with varying degrees of accuracy (10). Also, very strong evidence shows that animals and even people seem to "feel" something just before an earthquake, volcanic eruption, or other natural disaster (Randles 188). This is evidence of some kind of mental time travel illistrating the ability of the collective unconscious to travel from "now" to "later" and then come back to "now" and educate us (Time Slips 2862). A good psychiatrist most likely could explain away all the evidence as simply tricks of the mind, but some believe that this form of time travel actually works and could be utilized if only more study were done (Randles 160).
Ghosts can also be categorized as a form of nonphysical time travel coming typically from the past into the present. It is felt by some that ghosts are short clips from the film of time that appear at certain places over and over again (Randles 54). A very famous ghost picture features the image of a man's long dead mother sitting behind him (Spirit Photography 174-75). This would imply that that image somehow traveled through time to meet him. How this energy travels through space and time, no one really knows. Ghosts and profetic dreams belong to the paranormal side of science. Unfortunately, little is known about this part of the world and little can be said about its basis on time travel. The more exciting developments of time travel, however, are based in the more "stable" realm of physics.
Physical time travel into the future is relatively simple. Einstein's theory of general relativity tells us that time is relative and the faster a person traveles the less that person ages (Schwinger 48). Let's say we have a set of twins Herbert and Helga. Herbert jumps onto a spaceship and heads at half the speed of light to Alpha Centari, our nearest solar neighbor. Helga, on the other hand, being her timid self, stays home. When Herbert returns home, he will find that he has aged less than Helga. To better explain this let's say that Herbert and Helga both keep identical clocks with them. This clock shoots a beam of light from its base up one meter to a mirror that triggers the light to shut off. The light comes on each second. The light from Helga's clock, which is relativly stationary on Earth, travels up one meter. However, the light from Herbert's clock not only travels up one meter but it also must travel forward at a rate of 150,000 Km per second. It takes longer for the light to reach the mirror for Herbert's clock than for Helga's. It can be said that time is slowed down for Herbert. This phenomena is also true for mechanical clocks and biological beings like ourselves. Theoretically, if we could accelerate someone to the speed of light, that person would cease to age and could travel as far into the future as he or she wished (Gribbin 154). Unfortunately that person would be stuck in the future without the ability to return to their "present", and the theory of general relativity tells us that it's impossible to accelerate matter to the speed of light.
There are places in the universe, though, that would be more than happy to accelerate matter to near the speed of light. These places are called black holes. Black holes are collapsed stars with a point in the middle of infinite density called singularity. Surrounding the singularity at a distance is the event horizon. Anything that crosses the event horizon is destined to be sucked into the hole and can never get out (Ferguson 38). If our intrepid space travelers were to orbit closely to a black hole they could complete a journey of just a few hours, return back to their space station and find that they had actually been gone for years. The travelers could repeat this process as many times as they wanted to, jumping forward centuries, even millennia.
If, while our travelers were on their journey, they sent a probe into the black hole, that probe would, before it was destroyed miliseconds later, witness the rest of the history of the universe. This happens because the light comeing into the black hole recieves so much gravitational energy that the wave lengths of the light come closer together and the probe views the outside universe as moving infinatly fast. Conversely, the travelers would watch the probes descent slowl as it fell into the black hole until it reached the event horizon in which case the probe would appear to be suspended there for all eternity. This is caused from the effect gravity has on light as it tries to escape the heavy gravity that surrounds the black hole. The light loses energy so its wave lenght is stretched allowing for the image of the probe to remain (Gribbin 226). The affect that gravity has on light is the same effect that gravity has on time.
Closed Timelike Loops (CTLs) offer more possibilities and controversy (Time Loops 2853). A CTL offers a traveler the ability to leave one place and time, travel around another place and time and then return to the same place and time that the traveler originated from. The problem with this kind of time travel stems from the situation where the traveler goes into the past and kills his own grandmother, thus preventing the birth of his mother and ultimately his own birth. However, since he isn't born he can't travel back in time and kill his grandmother, so his mother is born and she gives birth to our traveler who goes back in time and so on and so forth on into infinity (Gribbin 185). Simply put, a CTL has the potential to violate causality. Causality states that causes always precede their events. First the trigger is pulled, then the gun goes off, never the other way around (Davies 249).
There are a few possible ways to get around causality in time travel. One states that the past is set in a rigid pattern that cannot be changed and that our traveler will fail at killing Granny. A variation of this states that if Granny is killed then the traveler's grandfather will marry Granny's sister and the future will be changed only slightly. The many worlds theory states that whenever a change can be made the universe splits and creates a place where Granny is killed and a place where Granny lives (Gribbin 186-89).
Wormholes are our most reliable source of CTLs. Black holes are our most reliable source of macroscopic wormholes. The mathematics used to describe a black hole are simple algebraic equations. These equations actually describe the connections between two regions of space time, two universes (Gribbin 156). If the black hole is spinning, and most certainly are, then the singularity isn't a point but a ring. Through this ring lies another universe, or more likely another place in our own universe. A traveler who dived through the black hole and orbited the ring singularity in an appropriate manner could maneuver himself into the past. But he could not exit the black hole in the same region of space time that he started from (Gribbin 161). If a black hole rotated with sufficient speed then it would throw off its event horizon and leave behind a naked ring singularity (Hawking 185). With no one way horizons to stop the traveler, he could travel to any point in space time continuum and then return to the place he started from at the same time, or even before he left (Gribbin 164).
The problem with any form of time travel involving black holes is that the possibilities of a naked ring singularity's existence are almost nothing. And the closest black hole we could use for such recreational activities as skipping into the future is the theoretical black hole at the center of our galaxy which happens to be a mere thirty-thousand light years away (Gribbin 149).
There are ways for us to time travel without black holes. One man, Frank Tipler, thinks he knows how to construct a working time machine. By creating a cylinder ten times as long as it's width with the mass of our sun and the density of a neutron star and spinning it at a rate of twice every millisecond a usable time machine can be produced (Ways to Travel Thought Time 77). This time machine would swirl space time around it so much that a space ship flying beside it would enter a region of pure space where it could go anywhere it wanted with no motion through time. By directing the space ship towards one end of the cylinder the ship would travel into the past. By reversing it's course the ship would travel past the present and into the future (Gribbin 203). The ship could, realistically, travel anywhere and any time it wanted to and still return home at the same time it left. Just like the ship that traveled around the naked ring singularity only without the black hole. Evidence has shown that paranormal communications through time may exist. And physics certainly tells us that time travel is not impossible. In fact there are many ways to travel through time that give our traveler full range of space and time on into infinity. Unfortunately the time when a mad scientist can construct a time machine in his basement to transport himself to the past or future is a long way off, but certainly not impossible.
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